The isomerization of saturated hydrocarbons (paraffins) to the corresponding branched isomers is an important process, for example, for increasing the research octane number (RON) of gasoline, in order to improve the combustion properties thereof.
Branched cyclic hydrocarbons can isomerize with ring enlargement to less branched cyclic hydrocarbons; one example is the rearrangement of methylcyclopentane (MCP) to cyclohexane. These reactions are catalyzed by strong Lewis acids or strong Brønsted acids.
The isomerization of saturated hydrocarbons with solid aluminum chloride has been known for some time. A frequently used accelerating additive is HCl (e.g. U.S. Pat. No. 2,493,567, U.S. Pat. No. 3,233,001, U.S. Pat. No. 5,202,519). Problems in these processes are the long-term stability of solid aluminum chloride and the removal thereof.
US 2003/0109767 A1 (Vasina et al.) reports that ionic liquids consisting of a nitrogen-containing heterocyclic or aliphatic cation and an anion which derives from a metal halide can be utilized for isomerization of paraffins in the direction of more highly branched paraffins at relatively low temperatures.
Cyclic hydrocarbons with a tertiary carbon atom as additives, such as methylcyclohexane and dimethylcyclopentane, according to EP 1 403 236 A1 (Haldor Topsoe A/S), increase the selectivity with regard to the formation of more highly branched hydrocarbons from less branched or unbranched hydrocarbons.
Ionic liquids consisting of n-butylpyridinium chloride and aluminum chloride can be utilized in order to isomerize methylcyclopentane and cyclohexane: V. A. Ksenofontov, T. V. Vasina, Y. E. Zubarev, L. M. Kustov, React. Kinet. Catal. Lett. 2003, Vol. 80 (2), pages 329-335.
The utilization of combinations of aluminum halides and copper(II) chloride or copper(II) sulfate, in each case in a ratio of 1:1, for the isomerization of pentane is reported by Ono et al. in Chem. Lett. 1978, 1061-64; J. Catal. 1979, 56, 47-51; and J. Catal. 1980, 64, 13-17.
The isomerization of paraffins with aluminum chloride and copper(II) chloride in molar ratios of 2:1 to 3:1 is described in U.S. Pat. No. 5,202,519 A (Phillips Petroleum Comp.), wherein the selectivity with regard to the isomerization is lower than the selectivity with regard to disproportionation.
US 2003/0181780 A1 (Herbst et al.) reports the isomerization of paraffins to more highly branched hydrocarbons with ionic liquids which comprise aluminum halide and additionally metal halides, for example copper(II) chloride, iron(III) chloride and molybdenum(V) chloride.